The present invention generally relates to semiconductor device fabrication tools and, more particularly, to such tools utilizing an electron cyclotron resonance plasma for processing device surfaces, e.g., as in plasma etching or thin film deposition applications.
In a conventional electron cyclotron resonance (ECR) plasma reactor, microwave power is used to produce the plasma in the presence of a magnetic field. The magnetic field strength and the microwave frequency are selected to bring about electron cyclotron resonance inside the plasma. This condition results in efficient microwave energy excitation of the plasma electrons.
Existing ECR reactors used in plasma processing lose a significant part of the plasma generated in the resonance zone to the reactor walls or to other destinations outside the desired target (device workpiece). Many of the magnetic field lines that pass through the resonance zone also intersect non-target areas, such as the reactor chamber walls. Since the charged plasma particles follow the magnetic field lines to a large extent, there is a resultant significant power loss to the walls, as well as the possibility of contamination due to the plasma bombardment of the walls. In one instance, an axial magnetic field is provided by a solenoid coil positioned around the plasma generating chamber as described in U.S. Pat. No. 4,915,979, issued Apr. 10, 1990 to Tomoaki Ishida et al. In another case, multipole magnets surround the chamber to provide a cusp-shaped magnetic field as taught in U.S. Pat. No. 4,745,337, issued May 17, 1988 to Michel Pichot et al. In a third reference, an axial magnetic field is used in combination with a cusp-shaped magnetic field as described in the paper "Si Etching With Low Ion Energy In Low Pressure ECR Plasma Generated By Longitudinal and Multipole Magnetic Fields" by H. Shindo et al, Proceedings of Symposium on Dry Process, Oct. 30-31, 1989, Tokyo, pg. 21-26, sponsored by the Institute of Electrical Engineers of Japan.
Each of the above described magnetic field configurations, as employed in the prior art, generate an ECR zone which is contiguous to the chamber walls of the plasma generator. Inasmuch as the charged plasma particles tend to follow the magnetic field lines that pass through the plasma, many of the plasma particles are either directly intercepted by the contiguous chamber walls or reach the walls downstream of the ECR zone or otherwise fail to reach the intended target or workpiece within the plasma processing tool. The result is a costly reduction in tool efficiency and a possible contamination of the work piece due to ionic bombardment of unintended targets.
Referring to a prior art plasma generator exemplified by FIG. 1, a pair of solenoid coils 41 and 42 surround a cylindrical plasma reaction chamber 43. Coils 41 and 42 are energized to provide a magnetic field directed axially along chamber 43 which interacts with a source of microwave excitation (not shown) to excite gas 44 to produce a plasma within ECR zone 48 in a manner well understood in the art as described in the U.S. Pat. No. 4,915,979. Magnetic field lines 46 guide the plasma particles generally toward wafer target 47, although many of the plasma particles are intercepted by the walls of chamber 43 contiguous to ECR zone 48 and others of the particles are guided by some of the magnetic field lines to destinations away from wafer 47. Obviously, the overall tool efficiency with which the surface of wafer 47 is treated by the plasma particles is reduced to the degree that the particles are misdirected. Thus, a plasma source design that alleviates the foregoing power loss and contamination problems would allow for more efficient operation than is available using conventional reactors.